Mathematical Bold Small Mu

Use \boldsymbol{\mu} in a LaTeX equation.

Load the amsmath package; with the bm package, \bm{\mu} is another option. These commands ask the document’s math font for bold Greek mu. In standard LaTeX, \mathbf{\mu} does not reliably bold lowercase Greek letters.

What does bold mu mean?

Its equation, not the letter alone, defines the quantity.

Bold lowercase Greek letters commonly mark vectors in physics and applied mathematics. Authors use the weight to distinguish a variable, but each formula defines what this mu represents.

Greek mu descends from Phoenician mem, a consonant letter for /m/ whose name is linked with water. Greeks adapted Phoenician writing around the 8th century BCE, borrowing its letter names and order and adding vowel signs. The earliest surviving Greek inscriptions date to that century and already show local forms; lowercase Greek shapes developed much later in handwriting.

Why did bold mu turn into plain μ?

Compatibility normalization removes the mathematical style.

NFKC and NFKD are text-cleanup standards. They map mathematical letters to ordinary Greek or Latin letters, so 𝛍 becomes μ and loses its encoded bold distinction.